FIG. 7 is a block diagram illustrating an electric arrangement of an optical transmitter 1, which is a typical driving circuit in the prior art. The optical transmitter 1 is shown in FIG. 9 of Japanese Publication of Patent, Tokukouhei, No. 4-64216 (published on Oct. 14, 1992). The optical transmitter 1 drives a light emitting element 2, which is a semiconductor laser or a light emitting diode. An input signal s1 is supplied from a signal source 3 to a buffer amplifier 4, which amplifies a difference between the input signal s1 and a reference voltage vref1, which are inputted into the buffer amplifier 4. One input terminal of an OR circuit 5 directly receives an output signal s2 of the buffer amplifier 4, while the other input terminal of the OR circuit 5 receives a signal s2d, which is generated by delaying the output signal s2 by means of a low-pass filter constituted of a resistor r and a capacitor c. Therefore, the OR circuit 5 outputs a driving output s3 during a period in which at least one of the signals s2 and s2d is higher than a predetermined reference voltage vref2 (not shown).
The OR circuit 5 also generates a differential output s3a which is inversion of the driving output s3. The outputs s3 and s3a are respectively supplied to bases of transistors q1 and q2, which composes a differential current driver. Emitters of the transistors q1 and q2 are equally grounded via a constant current source 6. A collector of the transistor q1 is connected, via the light emitting element 2, to a power source line of a high level Vcc, while a collector of the transistor q2 is directly connected to the power source line of the high level Vcc. With this arrangement, the transistor q1 is turned ON only during the period in which the output signal s2 corresponding to the input signal s1, and the delay signal s2d are higher than the reference voltage vref2. Thereby, the light emitting element 2 is turned ON so as to emit output light pho, when a collector current i of the transistor q1 exceeds an oscillation threshold value ith, in case where the light emitting element 2 is a semiconductor laser.
FIG. 8 is a waveform chart explaining an operation of the optical transmitter 1 having the aforementioned arrangement. Assuming that delay time caused by the buffer amplifier 4 is negligibly small, the output signal s2 of the buffer amplifier 4 has a duty ratio of 50%, for example, when the input signal s1 received by the buffer amplifier 4 has a duty ratio of 50% and an average value that is equal to the reference voltage vref1. When the output signal s2 is delayed by the low-pass filter composed of the resistor r and the capacitor c, the output signal s2d, which rises and falls gently with a certain time constant, is outputted. When an OR operation of the signals s2 and s2d is carried out, a voltage pulse having a duty ratio greater than that of the input signal s1 is outputted as the driving output s3. Further, delayed rise of the light emitting element 2 gives the output light pho an actual duty ratio smaller than that of the driving output s3.
Here, FIG. 9 shows a relationship between a driving current iLD and an optical output pLD, in case where the light emitting element 2 is a semiconductor laser. The semiconductor laser has such a non-linear property that laser oscillation is started when the driving current iLD exceeds the oscillation threshold value ith, thereby suddenly increasing a power of the optical output pLD so as to start the emission of the light from the laser.
Moreover, FIG. 10 shows a relationship between a driving current iLED and an optical output pLED, in case where the light emitting element 2 is a light emitting diode. In the light emitting diode, the optical output pLED increases in proportion to the driving current iLED. However, when the increase of the optical output pLED reaches a certain level, a rate of the increase is decreased.
As described above, in either cases, there is a delay between a time at which the light emitting element 2 is started to drive, and a time at which the light emitting element 2 reaches a predetermined luminance level. Because of this, the output light pho has an actual duty ratio lower than that of the driving output s3.
For this reason, the prior art requires such an arrangement in which the low-pass filter is provided for giving the input signal s1 a large duty ratio so as to adjust a time constant thereof, thereby obtaining the output light pho having the duty ratio (of 50% in FIG. 8, as described above) equal to that of the input signal s1.
Here, the above arrangement will be explained in more detail. The time constant of the low-pass filter determines how much the duty ratio of the driving output s3 increases with respect to the duty ratio of the input signal s1. Moreover, the time constant of the low-pass filter is so adjusted that the time constant can cancel out the reduction in the duty ratio of the output light pho with respect to the duty ratio of the driving output s3. With this arrangement, it is possible to obtain the output light pho having the duty ratio (of 50% in FIG. 8, as described above) equal to the input signal s1.
However, finished products of the light emitting element 2 have production unevenness among them in terms of its light emission delay time (a length of time of the delay in the light emission). Therefore, the light emission delay time is measured for each light emitting element 2 to use, so as to adjust the time constant of the low-pass filter.
In such prior art, the duty ratio of the actual output light pho is adjusted to be equal to that of the input signal s1. However, only an cutoff frequency (fc=1/(2πrc)) determines a compensation amount of a pulse width of the output light pho (how much the pulse width of the output light pho is to be compensated). Therefore, in case where the low-pass filter is integrated on the integrated circuit, there is a drawback that the compensation amount cannot be adjusted. Especially, a difference between the duty ratios becomes unignoring when the input signal s1 has a high frequency.
The following will explain how the frequency of the input signal s1 relates to the duty ratio, by discussing an example arrangement in which the light emitting element 2 has a delay of 500 psec so that the delay cannot be compensated. As an example where the input signal s1 has a low frequency, assume that a transmission rate is 250 Mbps, the input signal s1 has the duty ratio of 50%, a pulse width of 4 nsec, and a pulse cycle of 8 nsec. In this case, the output light pho has a pulse width of 3.5 nsec, and a duty ratio of 43.75%. On the other hand, even with the same arrangement (in which the delay of the light emitting element 2 is 500 psec), when the input signal s1 has such a high frequency whereby the transmission rate is 500 Mbps, and the input signal s1 has the duty ratio of 50%, a pulse width of 2 nsec, and a pulse cycle of 4 nsec, the output light pho has the pulse width of 1.5 nsec and the duty ratio of 37.5%.
Therefore, if the adjustment of the compensation amount is impossible as previously discussed, it is a problem that an optical communication apparatus of higher speed cannot be realized without deteriorating its yield. Moreover, if a resistor r or a capacitor c of the low-pass filter for the adjustment of the compensation amount is mounted externally on the integrated circuit, a line for the signal s2 is exposed out of the integrated circuit, thus being more susceptible to disturbance noise. Thus, this arrangement has a poor jitter property of the signals. Furthermore, in order to increase the compensation amount, the low pass filter needs to have a large time constant. However, it is a drawback that such large time constant makes the jitter larger because the jitter depends on signal frequencies.